Bleaching of medium consistency pulp with ozone without high shear mixing

ABSTRACT

Methods are disclosed for bleaching medium consistency cellulose pulp including generating a stream of ozone-containing gas having an ozone concentration of at least 10% by weight and radially injecting the stream of ozone-containing gas into a stream of cellulose pulp flowing through a reactor so as to provide a stream of bleached cellulose pulp. A low to medium intensity mixer operating at no more than 800 rpm may be used, downstream from the injection site.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 09/763,221, filed May 15, 2001, the disclosure ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method of bleaching mediumconsistency pulp with an ozone-containing gas. More particularly, thepresent invention relates to the proper utilization of the very fastreaction of ozone, by providing efficient but pulp-preserving mixingimmediately on introducing a substantial amount of ozone into the pulp.

BACKGROUND OF THE INVENTION

A number of methods for the bleaching of pulp with ozone are known inthe art. These methods have developed with the goal of carrying out thebleaching stage with medium consistency pulp, i.e. having a consistencyof about 7 to 16 percent.

Generally, ozone bleaching of medium consistency (MC) pulp according tocurrent practice can be described as ozone generation followed bycompression before introducing the ozone containing gas into the MC pulpflow. The gas-liquid-fiber suspension is thus vigorously treated in oneor several high shear mixers before the suspension is led into ableaching tower. The ozone may be introduced at several points along thepulp stream. Vent gases must be treated because of excess ozone which iscarried over.

The principle thus described may well be a result of the application ofoxygen bleaching methods. Oxygen, however, operates at a much slowerrate, and the temperatures which are used are significantly higher thanthose employed in ozone bleaching.

Typical and frequent problems arise from the difficulty in keeping thesuspension uniform. Segregation into two-phase flow readily occurs, andthe ozonization rate drops significantly (to 1 or even 0.1% of itsoptimum rate). This is a dominant problem, which may be reduced by usinga higher quality ozone, resulting in less gas void and consequently lessneed for vigorous mixing. A typical solution in the present state of theart is the use of more than one mixer. This does not, however, eliminatethe problem, and by applying more shear forces to the pulp, the strengthproperties of the resulting product are severely affected.

A basic problem with such mixers is the short residence time, and ifmixing time is increased, undesired backmixing may occur.

After leaving the mixers, the gas-pulp suspension rapidly segregatesinto two-phase flow having a relatively small gas-liquid interface perunit volume. The chemical consequences of this are low capacity and anon-uniform bleaching result. Obvious evidence of this phenomenon is thesignificant ozone surplus often remaining after the bleaching stage,representing both a hazard and an economical loss.

A pulp bleaching method comprising introduction of high pressure ozonein a carrier gas into a pulp stream with vigorous mixing and subsequentremoval of carrier gas is disclosed in, e.g. European Patent No. 511433. The major issue in this patent is the removal of gas from the pulpafter injection into the mixer and the reaction is said to take placeessentially within ten seconds in a vertical reaction vessel situatedimmediately following the fluidizing mixer. Gas at about 10 to 13 bar,containing about 3 to 10% ozone by weight (6.8 vol %) is used.Preferably, the gas-pulp mixture is carried in a horizontal pathfollowing the vertical reaction step to effect separation of the largeamount of carrier gas involved.

Austrian patent application no. 2203/92 describes a method whereinmedium consistency pulp is treated with an ozone-containing gascomprising more than 120 g O₃/normal m³ gas (5.6 vol %) whereby the gasis introduced as fine bubbles with a low differential pressure(preferably less than 1 bar). It is considered that using gas with ahigh ozone content, a sufficient amount of ozone can be suspended intothe gas to achieve the desired bleaching. Further, Austrian patentapplication no. 2203/92 discloses the use of mixers with or withoutfluidization effects, and of an ozone reaction stage subsequent to themixing stage, as well as additional ozone addition stages, withdegassing stages in between. Characteristically, the highly concentratedozone is introduced in static mixers at several points, possiblyremoving the inert carrier gas (normally oxygen) between stages, and thefinal reaction between ozone and fiber takes place in a bleachingreactor, typically of the traditional upflow type.

A common feature of several other publications disclosing ozone bleachprocesses for medium consistency pulp is the use of fluidizing mixers inconnection with the injection of ozone-carrying gas, and the use ofsubsequent, relatively extended reaction stages and gas separation.

In chemical process terms, MC ozonization can be described as ozonemolecules in a gas phase that must be transported to the vicinity of thefiber and react with the fiber or other substrates. The ozone mustdiffuse through the gas-liquid interface, through the liquid to thefiber. The applied mixing affects the size and the relative velocity ofthe gas bubbles, as well as the amount of fiber-liquid interface. Therate limiting step completely dominating the interaction of ozone withthe fiber material is the transport of ozone through the gas-liquidinterface. The gas-liquid transfer rate in a given volume is heavilydependant on the bubble size, i.e. gas-liquid surface area m² gas/m³suspension, and on the partial pressure of ozone. Other rate limitingsteps, like diffusion in the fiber material itself, are determined bythe nature and the consistency of the pulp, which is primarily affectedby the temperature.

Due to its dependency on mass transfer, the reaction rate of ozone is,theoretically and empirically, first order.

The high gas void, i.e. the low concentration of ozone generated by mostpresent ozone generators, limits the possibilities to improve thesituation. Reduced gas void in subsequent generations of ozonegenerators will reduce the need for mixing and reduce energyrequirements, as well as the size of the equipment. Higher ozoneconcentrations will also increase the ozonization rate.

Consequently, the following are characteristics of efficient processsolutions:

-   -   the residence time distribution (RTD) must follow a plug-flow        pattern (in contrast, backmixing commonly occurs in dynamic        mixers), which requires a special reactor geometry to avoid        backmixing e.g. appropriate turbine and baffles. High intensity,        high shear mixing is detrimental;    -   mean residence time in transfer/mixer/reactor must match        transport and reaction times for complete conversion of ozone;        consequently reactor diameter, shape and rotation rate of a        possible turbine must match flow rate; and    -   all ozone should be introduced in one step.

A low intensity mixer is sufficient for bleaching using moderate ozoneconcentrations, as long as the other criteria for ozone use arefulfilled. As the ozone concentration increases, the need for mixingdecreases, and a static mixer is sufficient for distributing the ozonein the pulp. At sufficiently high ozone concentrations, a mixer is notnecessary.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other objects havenow been realized by the invention of a method for bleaching mediumconsistency cellulose pulp comprising providing a stream ofozone-containing gas and radially injecting the stream ofozone-containing gas representing a single ozone addition into a streamof cellulose pulp flowing through a reactor, so as to provide a streamof bleached cellulose pulp, the reactor being operated in plug flow modeat a pressure of at least 3 bar and at a temperature of 50° C. or lower.Downstream from the ozone injection site, a low to medium intensitydynamic mixer operating at no more than 800 rpm may be used. The methodaccording to the invention may include the use of static mixers, e.g.baffles. These may preferably be used at higher ozone concentrations.Preferably, the ozone concentration is at least 10 percent by weight;more preferably at least 15 percent by weight; and most preferably about20 percent by weight. The expression “percent by weight” refers to theweight of ozone in the feed gas mixture.

In a preferred embodiment the method includes generating the stream ofozone-containing gas from a mixture of pressurized oxygen and at leastone other gas or liquid.

In accordance with one embodiment of the method of the presentinvention, the method includes radially injecting the stream ofozone-containing gas at a pressure of at least 7 bar into the stream ofcellulose pulp.

In accordance with another embodiment of the method of the presentinvention, the method includes radially injecting the stream ofozone-containing gas into the stream of cellulose pulp from a pluralityof nozzles adapted to direct the ozone.

High-concentration, high pressure ozone is introduced into the pulpline, whereby conditions approaching plug flow are achieved, a highconcentration of ozone is reached with a mass transfer area in thesuspension which is sufficient for effective delignification.

According to one aspect of the present invention, the ozone isintroduced using effective injection nozzles providing for the efficientdispersion necessary for obtaining a uniform distribution as well assufficient mass transfer area to overcome the rate-limiting masstransfer threshold present in methods according to the prior art. Thus,the need for fiber-destroying high shear fluidizing mixers is removed.

According to a further aspect of the invention, a reactor for bleachingmedium consistency pulp according to the above defined method isprovided. The reactor comprises a vessel and means, for radicallyinjecting ozone into a pulp stream flowing through the vessel, e.g.nozzles.

According to another aspect of the present invention, a dynamic low tomedium intensity mixer is provided in the pulp stream downstream of theozone injection site. Such a mixer delivers to the pulp stream amountsof energy which are well below fluidization energies, and does notmechanically affect the fiber.

Preferably, the low to medium intensity mixer is of the helix screw typewith high pitch in the center, preferably 20 to 45 degrees, and lowerperipheral pitch, preferably 5-30 degrees.

Baffles can be either of the helix-type on the vessel inside with anegative pitch, preferably 5-45 degrees with respect to the mixer, orflat discs dividing the reactor volume, preferably in at least 5segments (compartments); or both baffle types at the same time. If flatdisc baffles are used, a double helix-mixer with opposite signs on thepitch may be used.

According to another aspect of the present invention, a static mixer isprovided in the pulp stream immediately downstream of the ozoneinjection site.

Preferably, the process is operated at temperatures below 25° C. toreduce the decomposition of ozone to negligible values. At temperaturesabove 40° C., the self-decomposition of ozone increases significantly,and the process according to the invention should be operated at atemperature not higher than 50° C.

Higher temperatures will lead to higher requirements of ozone charge,but also to shorter residence times and smaller bleaching reactors.

The mean residence time of the pulp in the reactor may vary betweenabout 10 to about 150 s, and longer time is required for higher ozonecharges and for lower ozone pressure and concentration. Preferably, themean residence time is between about 15 and about 40 s.

The mixing efficiency is not sensitive to the gas void level and thereactor can handle gas voids from 10 to 50 vol %.

With the aid of recent technology, as disclosed, e.g., in Swedish PatentApplication No. 9502339-6, ozone with a concentration of up to 18 to 20%by volume may be generated. References to concentrations as high as 300g O₃/Nm³ have been made in prior art publications (e.g. EuropeanApplication No. 426,652, with a priority date of Oct. 30, 1989), butsuch concentrations have not been technically feasible until recently.Using a high ozone concentration (300 g/m³ and higher) and at highpressure (10 bars and higher) together with a proper injectiontechnique, the reaction between ozone and fiber can now take place at arate such that the subsequent use of an upflow bleach tower is notnecessary. The gas pressure is obtained by using precompressed oxygen,optionally mixed with other gases or liquids (e.g. argon) to maintain asuitable conductivity for ozone generation.

Oxygen is the most common carrier gas used for ozone. Highlyconcentrated ozone is usually considered to be an explosion hazard. Asthe ozone generating technology has developed, the accepted limit forstable oxygen-ozone mixtures has been repeatedly pushed upwards, and itappears that no absolute concentration limit for the safe handling ofozone has yet been established. Thus, use of very high ozoneconcentrations may yet be possible, which further facilitates use ofmethods according to the present invention. According to the presentinvention, the concentration of ozone in the gas introduced into thepulp stream is sufficient for achieving bleaching without anyfiber-destroying mechanical impact.

The initial distribution of highly concentrated ozone into the pulp isof importance, for the selectivity, as the carbohydrate component itselfmay be attacked by ozone if exposed for an extended time. The absence ofbackmixing, as may occur in high shear mixers, and the presence of plugflow conditions counteract this phenomenon.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be more fully appreciated with reference tothe following detailed description, which in turn refers to the drawing,in which;

FIG. 1 is a graphical comparison between the changes in reaction ratesagainst time in a prior art ozone pulp bleaching process using a mediumconsistency mixer, and a process according to the present invention.

DETAILED DESCRIPTION

The present invention may be appreciated with reference to the followingspecific examples;

EXAMPLE 1

Ozone-carrying gas having a pressure of about 15 bar and an ozoneconcentration of 14% by volume is introduced into a medium consistencypulp line carrying 1000 tons/day by means of a collar of radiallyarranged nozzles. Preferably, the nozzles are arranged to direct the gasradially into the pulp flow, essentially in a direction perpendicular tothe pulp flow. A number of nozzles sufficient for evenly distributingthe gas must be used. On this production scale, 186 nozzles with aninlet diameter of a maximum of 1 mm may be used.

A sufficient mean residence time (10 to 40 seconds) must be allowedbefore any other disturbing action to the pulp.

EXAMPLE 2

A medium intensity (low-shear) mixer is adapted into the pulp stream ofthe previous example, preferably immediately following the gas injectionsite. The mixer turbine is preferably a double or multiple screw withblade angles and rotation rate balanced to maintain the plug flowresidence time distribution (RTD) and giving good radial mixingefficiency. The center blade has a steeper angle than the outer screwblade. Alternatively, porous metal injector devices for introduction ofozone can be arranged peripherically or on the turbine.

FIG. 1 shows a comparison between a system employing a traditionalmedium consistency mixer with a very high capacity for a short intervaldropping rapidly to zero, compared to a system according to the presentinvention, with a moderately high capacity kept constant for a longperiod. The dotted line represents state-of-the-art traditional mediumconsistency mixer technology. The first, steep section shows the effectof the mixer with high reaction and uniform distribution. The low ratesection shows the effect of the corruption of the gas-suspensioninterface. The reaction takes place with a nonuniform distribution andthe pulp is mechanically stressed by high shear mixing.

The solid line represents a system according to the present invention.Throughout the process, a moderately fast reaction is taking place in amildly stressed pulp and with a uniform distribution of ozone. TABLE 1Present Calculus Base Units Conventional Modern invention Pulpproduction ton 1000 1000 1000 OD/day Consistency % 10 10 10 Ozonepressure bar 9 9 15 Ozone w % 10 14 20 concentration vol % 7 10 14 Ozonecharge kg/ton 5 5 5 (3-5) OD pulp Ozone generator kg/h 208 208 208 Ozonevolume flow m³/s 0.0146 Nozzle diameter m 0.001 Number of nozzles 186Process Process ° C. 40 40 40 temperature Process pressure bar 7 7 15Pulp Flow ton OD 42 42 42 pulp/h Volume Flow m³/h 375 375 375 MC pulpOzone gas charge m³/h at 234 165 53 actual press. Gas void* % 38 31 12Equipment Ozone Ozone No ozone compressor compressor compressor 1-3mixers 1+ mixers No mixer Bleach Bleach Small bleach tower tower reactor*Note:Gas void is proportional to process problems

Table 1 shows a comparison in numbers between a typical conventional MCbleaching system, a state-of-the-art system and a system according tothe present invention.

1. A method for bleaching medium consistency cellulose pulp comprisingproviding a stream of ozone-containing gas and radially injecting thestream of ozone-containing gas in a single ozone addition step into astream of cellulose pulp flowing through a reactor so as to provide astream of bleached cellulose pulp, wherein said reactor operates in plugflow mode at a pressure of at least 3 bar and at a temperature of 50° C.or less and said reactor does not include a high shear mixer.
 2. Themethod of claim 1 wherein said ozone-containing gas has an ozoneconcentration of at least 10 percent by weight.
 3. The method of claim 1wherein said ozone-containing gas has an ozone concentration of at least15 percent by weight.
 4. The method of claim 1 wherein saidozone-containing gas has an ozone concentration of at least 20 percentby weight.
 5. The method of claim 1 including radially injecting saidstream of ozone-containing gas into said stream of cellulose pulp bymeans of a plurality of porous metal injectors.
 6. The method of claim 1wherein the mean residence time of said pulp in said reactor is betweenabout 10 and about 150 s.
 7. The method of claim 6 wherein the meanresidence time of said pulp in said reactor is between about 15 andabout 40 s.
 8. The method of claim 1 wherein the stream ofozone-containing gas is generated from a mixture of pressurized oxygenand at least one other gas or liquid.
 9. A reactor for bleaching mediumconsistency pulp wherein said reactor comprises a vessel, means forradially injecting an ozone-containing gas into a stream of cellulosepulp flowing through said vessel, and said reactor further comprising amixer of the rotatable helix screw type having a first pitch in thecenter and a second pitch at the periphery, the second pitch being lowerthan the first pitch, downstream from said injecting means.
 10. Thereactor of claim 9 wherein the first pitch is in the range of 20 to 45degrees and the second pitch is in the range of 5 to 30 degrees.
 11. Thereactor of claim 9 including baffles on the vessel inner wall.
 12. Thereactor of claim 11 wherein said baffles are of the helix type, having anegative pitch in respect to the mixer.
 13. The reactor of claim 12wherein said baffles have a pitch in the range of 5 to 45 degrees. 14.The reactor of claim 11 wherein said baffles comprise flat disc bafflesdividing the reactor volume.
 15. A method for bleaching mediumconsistency cellulose pulp comprising providing a stream ofozone-containing gas and radially injecting the stream ofozone-containing gas in a single ozone addition step into a stream ofcellulose pulp flowing through a reactor so as to provide a stream ofbleached cellulose pulp, wherein said reactor operates in plug flow modeat a pressure of at least 3 bar and at a temperature of 50° C. or lessand said reactor includes a mixer operating at not more than 800 rpmdownstream from the injection site.
 16. The method of claim 15 whereinsaid ozone-containing gas has an ozone concentration of at least 10percent by weight.
 17. The method of claim 15 wherein saidozone-containing gas has an ozone concentration of at least 15 percentby weight.
 18. The method of claim 15 wherein said ozone-containing gashas an ozone concentration of at least 20 percent by weight.
 19. Themethod of claim 15 wherein said mixer includes a static mixer.
 20. Themethod of claim 15 wherein said mixer includes a dynamic mixer.
 21. Themethod of claim 15 wherein said mixer includes a rotatable helical screwmixer.
 22. The method of claim 15, including radially injecting saidstream of ozone-containing gas into said stream of cellulose pulp bymeans of a plurality of porous metal injectors.
 23. The method of claim15 wherein the mean residence time of said pulp in said reactor isbetween about 10 and about 150 s.
 24. The method of claim 23 wherein themean residence time of said pulp in said reactor is between about 15 andabout 40 s.
 25. The method of claim 15 wherein the stream ofozone-containing gas is generated from a mixture of pressurized oxygenand at least one other gas or liquid.